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Bonded anchors – application in hollow substrates

6 min

Chemical anchoring (also known as bonded anchoring) is one of the most universal and reliable fixing techniques. However, before you proceed with resin application, you should first consider the kind of substrate in which you are about to fix.

Hollow substrates will not only require a specific type of product, but also appropriate installation steps. Read on to learn more details. 

Operating principle of bonded anchors

Unlike conventional mechanical fixings, the operation of which is based on the forces of expansion, bonded anchors bond to the substrate by way of adhesion, i.e. the surface bonding of materials. How does a bonded anchor work? Resin adheres uniformly to both the substrate and the component being fixed (typically a threaded rod). For that reason, when using bonded anchors, you can avoid generating stresses in the material, which proves particularly important when installing in weaker hollow substrates. 

Hollow substrates – pros and cons

According to the most general classification, base materials can be broken down into solid concrete and masonry substrates. One of masonry materials (next to solid blocks) is ceramic hollow brick, which comes in the following variations: 

  • perforated brick
  • hollow clay tile
  • cinder block 

Hollow materials are made from clay, silt, and lightweight fillers (e.g. slag, expanded clay). Ceramic hollow bricks are used for the construction of single- and multi-family houses, public utility buildings, load bearing walls and partition walls. Being very versatile and popular, hollow substrates can be encountered not only during professional construction work, but also in simple home installation jobs. 

Ceramic hollow brick is characterised by low tensile and compressive strength. Being full of voids, this kind of base material displays lower strength: the more porous the material, the weaker it is. At the same time, hollow blocks show good thermal insulation properties, which makes them very popular in the construction industry. 

Bonded anchors for hollow substrates – which one to choose?

When choosing a bonded anchor, always make sure that the product you intend to use is suitable for the substrate in which you are about to fix. You can find detailed information on this matter in the product datasheet as well as in the relevant European Technical Approval (ETA). Importantly, not all bonded anchors are suitable for masonry materials. The most versatile polyester resins are definitely used most extensively, as they can be applied in non-cracked concrete, but also in solid as well as hollow bricks, hollow masonry units, concrete blocks, natural stone, or aerated concrete. The method of testing anchors in masonry substrates has been specified in European guidelines ETAG 029. 

Using polyester anchors in hollow substrates. Step-by-step installation instructions

If you are about to fix in hollow clay blocks using bonded anchors, keep in mind that the installation will generally not differ much from the application in concrete. The most important difference involves resin dispensing into the hole. Hollow materials require using a mesh sleeve, without which resin would simply flow down, making it impossible to effectively embed the rod in the hole. The weakest part of a load-generating system based on a hollow substrate is the substrate itself. For this reason, the anchor pressed through the mesh should work with the material and distribute the load evenly over as much of the substrate area as possible, thereby reinforcing the substrate structure at the anchorage point. This is a major advantage of resins, since conventional mechanical anchors can actually damage the material structure by applying expansion forces. 

Find out how to proceed when fixing in hollow substrates using bonded anchors:

Hole drilling

The first step in the installation of a bonded anchor is hole drilling. Importantly, if you are fixing in a masonry substrate, make sure to use a drill bit specifically designed for this type kind of base material. Also, do not use percussion drilling. 

Our recommendation

Are you wondering which drill bit to pick for masonry substrates? Our recommendation is the Brickdrill bit, available in many diameters and lengths. 

Remember that the hole diameter must be the same as that of the mesh sleeve into which the resin will be dispensed. At the same time, the hole must be larger than the diameter of the rod to be embedded. The recommended difference is 4 mm. 

Hole cleaning

Even though, when fixing in hollow substrates, resin must be dispensed into a mesh sleeve instead of directly into the hole, according to the relevant European Technical Assessment, step two of the bonded anchor application, i.e. hole cleaning, should not be skipped. The traditional hole cleaning method is 4x 4x 4x, which means using a hole cleaning brush four times, then blowing the hole through four times with a pump, and using the brush four times again. This is the only way to achieve optimal adhesion between the anchor and the substrate, and by that means, to make sure that the fixing will manage to transfer the required load. 

Resin preparation

Whichever kind of bonded anchor you are about to use (epoxy, polyester, hybrid), you need to prepare it properly before applying the resin. For this purpose, squeeze out approx. 10 cm of the mixture on a piece of cardboard, as much as needed to obtain a compound of uniform texture and colour. Only then is the product suitable for dispensing into the hole. 

Did you know that…?

A bonded anchor is composed of resin and a hardening agent. Both these ingredients must be mixed before application. This is achieved while the compound is being dispensed by means of a mixing nozzle attached to the cartridge.

Resin injection

The bonded anchor application proper should proceed as follows: dispense the compound evenly starting from the deepest section of the hole, gradually moving the mixing nozzle towards the front. In hollow substrates, when dispensing resin into the mesh sleeve, it should be filled to 100% (70% in solid substrates). 

Our recommendation

There are two types of mesh sleeve on Rawlplug’s offering: plastic and metal. The sleeves are available in a wide range of diameters, allowing you to easily select the accessory size to the installation job at hand. Additionally, metal sleeves can be cut to the desired length to match the depth of the hole in which resin is to be applied. 

Rod installation 

Once the resin has been dispensed into the mesh sleeve, you can proceed with the threaded rod installation. Keep in mind that the rod diameter must be 4 mm smaller than that of the hole. Insert the rod in a twisting motion. Importantly, the rod must be inserted before the resin starts to set. This interval is known as the installation time, which depends on the anchor type and the temperature of the substrate in which you are fixing. 

Bonded anchor loading 

Once the rod has been inserted into the hole, you must wait until the resin has completely set (this is known as the setting time of bonded anchors). Not until this time has elapsed can you apply any load on the anchor. 

Good to know

Wondering how to calculate the number of millilitres of resin needed to fill a single hole or to complete an entire project? Make use of our proprietary EasyFix program or the Resin Calculator from Rawlplug.

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